The symptom that points at termination
Lighting that flickers, steps to the wrong colour, or produces effects nobody programmed is often a wiring fault rather than a control fault. The three most common causes are an unterminated bus, a star instead of a chain, and too many devices on one run.
All three are physical-layer problems, and all three are decided before anyone opens the console.
What termination is for
A digital signal travelling down a cable reflects when it reaches an unterminated end, in the same way a wave reflects off a wall. On a DMX bus those reflections arrive back at the receivers and can be read as data, which is why they show up as erratic behaviour rather than as a clean failure.
The fix is a terminator: a resistor connected across the data pair at the far end of the run. The value is not arbitrary — it matches the cable’s characteristic impedance, which for DMX and RS-485 cable is 120 Ω. Matching the impedance absorbs the reflection instead of bouncing it back.
Termination belongs at the extreme end of the bus, furthest from the controller. Terminating in the middle does nothing useful, and terminating at the controller does not replace terminating at the far end.
Daisy chain, never a split
DMX is a bus, not a network. Devices are connected in a chain: data enters a fixture, and the same data continues out of its through port to the next fixture.
A Y-split — feeding two runs from one output — creates two unterminated branches and a discontinuity in the bus. When more than one run is genuinely needed, the correct device is a splitter, which presents a proper input and drives each output as its own bus.
The practical rule is simple: follow the data from the controller to the last device, and never let the path branch.
How many devices on one bus
The standard limits the load on a single bus by counting unit loads. A bus has a maximum number of unit loads, and once that is reached the run must be extended by splitting rather than by adding another fixture to the chain.
This is worth knowing because it changes the plan for a large rig. Beyond the limit, the answer is a splitter, not a longer chain — and each split output starts a new bus with its own budget and its own terminator.
Cable is part of the physical layer
DMX cable is specified as a shielded twisted pair with a characteristic impedance of 120 Ω. Microphone cable is also a shielded twisted pair and looks similar, but its impedance is not the same, so using it for a lighting run trades a known specification for an unknown one.
Impedance is a property of the construction — conductor spacing, insulation and shield geometry — which is why the cable has to be chosen for the job rather than by appearance.
Single-shielded and double-shielded constructions differ in how well the pair is protected, not in the impedance the bus expects. For runs near power distribution, dimmer racks or long cable trays, the additional shield layer is the relevant variable.
One case is worth separating before the checklist: a run built from Cat5e or Cat6. A network pair is specified at 100 Ω and the DMX bus expects 120 Ω, so no terminator value repairs that mismatch — DMX over Cat6 sets out which layer each cable belongs to.
Commissioning checklist
- The bus is a chain from the controller, with no branches.
- A 120 Ω terminator is fitted at the far end.
- The device count is within the bus limit, or a splitter is used.
- The cable is DMX-rated, not a microphone cable of similar appearance.
- The cable and terminators are on the same purchase list as the consoles and fittings.
Working through this list takes a few minutes and removes the three causes that produce the most confusing symptoms.
Summary
- A DMX bus needs a terminator at the far end, matched to the cable impedance.
- DMX cable is a 120 Ω shielded twisted pair.
- The topology is a daisy chain; splits require a splitter.
- The device count per bus is limited, and a splitter is the way to extend it.
Frequently asked questions
Why does my DMX lighting flicker or behave erratically?
Common physical-layer causes are a missing terminator at the end of the run, a split instead of a daisy chain, or too many devices on one bus. Check those three before suspecting the console.
What value is a DMX terminator?
It matches the cable’s characteristic impedance. DMX and RS-485 cable is specified at 120 Ω, so the terminator is a 120 Ω resistor across the data pair.
Can I use a microphone cable for DMX?
Both are shielded twisted pairs and can look identical, but the impedance specification is not the same. Use cable rated for DMX / RS-485.
Do I need a terminator if my run is short?
The standard calls for termination at the far end of the bus. Short, simple systems may behave acceptably without it, which is exactly why intermittent faults are hard to diagnose — termination removes the uncertainty.
Where does the terminator go?
At the extreme end of the bus, furthest from the controller — in the last device’s through connector.